In motor vehicles with engines which can be gas fuelled, the gaseous fuel is loaded and compressed in at least one cylinder or container. Pursuant to some safety regulations of the trade considered here, each gas cylinder on board a vehicle should be fitted with relief devices to discourage an undesirable combustive event if increases in internal pressure and/or temperature beyond a predefined threshold occur.
Each cylinder is fitted with a shut-off valve to stop flow into and out of the cylinder. Various types of cylinder valves are known which integrate several components, these valves are known as multipurpose valves. Generally speaking, such multipurpose valves comprise the aforementioned safety means and other devices pursuant to international regulations in the sector, such as for example, the excess flow device (EFD) which triggers to limit the exit of gas from the cylinder in the case of a difference of internal and external pressure above a predefined value. The excess flow device is fitted in the main duct through which the gas transits, while the pressure and temperature sensitive relief devices are directly connected with the inside of the cylinder and thus positioned at the end part of secondary ducts.
As such, the relief devices are of two types, namely those which are triggered by temperature and those which are triggered by pressure. Both are positioned along a gas release duct of the cylinder, to close the release duct in normal operating conditions.
The relief devices which are triggered by temperature comprise a heat-sensitive element confined in a respective seat or chamber in a solid body in association with an expulsion plunger. The heat-sensitive element normally keeps the release duct closed and only changes its state when a predefined safety temperature threshold is reached. Such change to the state of the heat-sensitive element then allows the release duct to be opened to empty the cylinder. The pressure relief devices generally comprise a rupture disk or other element that keeps the release duct closed and ruptures or otherwise opens when a predefined pressure safety threshold is reached, opening the release duct to empty the cylinder.
In most self-propulsion gas systems both relief devices are used.
Returning now to the main duct through which the flow transits, the excess flow device is found upstream of the manually operated flow shut-off device and upstream of the electric flow shut-off device, if present. One consequence of this arrangement, pursuant to legislation, is that in the event of maintenance operations emptying of the cylinder is required or desired, the aperture of the manual shut-off device generates a pressure differential such as to cause the excess flow device to cut in with resulting closure of the transit of gas.
Currently the only solution for emptying the cylinder is the partial manual aperture of the shut-off device; the partial aperture in fact allows the gas to flow out of the cylinder without the pressure differential reaching the cut-in value set for the excess flow device.
This solution has drawbacks as regards safety however. In fact, if a maintenance technician attempts to empty the cylinder by fully opening the manual shut-off device, the subsequent triggering of the excess flow device blocks the transit of gas leading the technician to believe that the cylinder is completely empty when instead there is still pressurised gas present inside it. A subsequent removal of the cylinder valve with pressurised fluid still present therein can result in an undesirable combustive event.
To overcome this serious problem, a proposal to revise the American legislation ANSI NGV3.1 has been drawn up, the main requisites of which are i) the multipurpose valve or the tank should be provided with an indication of the presence of pressurized gas therein; and ii) in the case of maintenance the exit passage of the gas from the tank should not be interrupted by the excess flow device or other shut-off devices placed downstream of the excess flow device.
A temporary solution to respond to the first request has already been proposed. Such solution consists of applying a pressure gauge to the multipurpose valve; in particular the pressure gauge is connected to one of the secondary ducts mentioned above. This solution is not however sufficient to provide for adequate safety of the system. The pressure gauge is in fact a cumbersome instrument which may be damaged in the case of impact and being connected, as such, to one of the secondary passages communicating directly with the inside of the tank a leak due to damage of the pressure gauge cannot be stopped by other devices.
An object of the present invention is to provide a safety device suitable for overcoming the above mentioned drawbacks.